Since the world energy crisis, it has become essential to be able to recover a maximum amount of hydrocarbons contained in underground formations. The most commonly used method consists in injecting, via an injection well, an aqueous fluid (generally water or brine). This fluid sweeps the underground formation so as to drive the hydrocarbons out of the pores of the rock where it is absorbed. Production wells allow a production effluent comprising a mixture of water, salts and hydrocarbons to be recovered. This method is known as enhanced oil recovery (EOR) method.
There are several enhanced oil recovery methods. When compounds are added to the fluid injected, which is also referred to as sweep fluid, the method is referred to as tertiary enhanced recovery. These chemical compounds are polymers, surfactants, alkaline compounds, or mixtures of such compounds. In relation to simple water or brine injection, the interest of the presence of a polymer is to increase the viscosity of the sweep fluid and therefore to improve the mobility ratio between the fluid injected and the hydrocarbons in place in the underground formation. The hydrocarbon recovery efficiency is increased as a result of a higher formation sweep efficiency. The polymers used in this method are generally polymers of high molecular mass used for their viscosifying properties.
Using polymers in tertiary enhanced recovery however poses some practical problems. In the production wells, an aqueous fluid/hydrocarbon mixture is recovered in form of an emulsion whose water/hydrocarbon ratio evolves as a function of the production time. After a certain operation time (referred to as polymer breakthrough), polymer mixed with the aqueous phase of the sweep fluid is recovered upon pumping in the production tubings. The polymer concentration in the production effluent is variable but however sufficient to disturb pumping thereof. Indeed, the polymer has the property of increasing the viscosity of the sweep fluid, it therefore also increases the viscosity and the viscoelastic properties of the production effluent. The higher the viscosity of the production effluent, the more the pressure drops and the pumping energy consumption increase during reservoir production. The viscosity increase of the production fluid can also lead to production tubing clogging, which requires to temporarily stop the development of the underground formation. The higher the viscous properties of the production effluent, the more it creates pumping difficulties, as well as problems with the other equipments necessary for surface treatment of the fluid.
The presence of polymer in the production fluid makes separation of the various fluids (oil/gas/water) and secondary treatment of the water more difficult. When the production effluent reaches the surface, it is treated in a surface unit. This unit allows the various fluids, gas, oil and water, to be separated. After this surface treatment, the hydrocarbons are ready to be refined. The water is treated and depolluted in order to minimize the discharge of toxic products into the environment. The presence of polymer in the fluids produced, as reported in publication SPE 65390 (2001) “Emulsification and stabilization of ASP Flooding Produced liquid” by Wu Di et al., can cause stabilization of the emulsions in the fluids produced and pose problems as regards the surface treatment methods, in particular the water/oil/gas separation and the secondary water treatment methods.
The goal of the present invention is to provide an enhanced recovery method that optimizes the stages of pumping, transport and surface treatment of the production effluent.